Composite dry vacuum pump having roots rotor and screw rotor

ABSTRACT

Disclosed is a dry vacuum pump for evacuating a processing chamber in semiconductor or display manufacturing device, or for evacuating the gaseous substance and/or the byproducts generated in process chamber. The dry vacuum pump does not need a partition wall between a roots rotor and a screw rotor. In the dry vacuum pump, a space is formed on the connecting portion to roots rotor among through the under sides of the roots rotor and the screw rotor to stay the object substance thereon.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application Nos. 10-2004-0078422, 10-2004-0078423, and 10-2004-0078431 filed on Oct. 1, 2004.

FIELD OF THE INVENTION

The present invention relates to a composite dry vacuum pump for evacuating a process chamber of a semiconductor manufacturing device and a display manufacturing device and the like, or for discharging gaseous substance and/or byproducts generated in the process chamber.

BACKGROUND OF THE INVENTION

Generally, a composite dry vacuum pump has been used for evacuating the process chamber a semiconductor manufacturing device and a display manufacturing device and the like, or for discharging gaseous substance and/or byproducts generated in the process chamber. Roots rotor, screw rotor or combinations thereof is used in the dry vacuum pump as described above.

Currently, the composite dry vacuum pump comprises at least one roots rotor having at least one lobe and at least one screw rotor so as to keep a process chamber perfect vacuum and reduce power cost requirement. A roots rotor is used in inhaling and compressing the byproduct including gaseous substance generated in process chamber, and a screw rotor is used in evacuating gaseous substance and byproduct inhaled by roots rotor outside the dry vacuum pump. These rotors are operated in sealed state to keep the process chamber vacuum.

In general, a partition wall is provided between a roots rotor end and a screw rotor end so that the byproducts do not hamper rotor's rotation and move smoothly from a roots rotor end to a screw rotor end. A representative example of these configurations is disclosed in U.S. Pat. No. 5,549,463 assigned entirely to Kashiyama Industry Co., Ltd. (Hereinafter refer to FIG. 7).

According to this patent, as illustrated in FIG. 7, a dry vacuum pump comprises a pair of roots rotors 102, 103 and a pair of screw rotors 105, 106. A pair of roots rotors 102, 103 and a pair of screw rotors 105, 106 are rotated by a driving motor 104. In more details, the driving force generated by a driving motor 104 is transmitted entirely to a pair of roots rotors 102, 103 and a pair of screw rotors 105, 106 with three gears, that is, a drive gear 124, an idle gear 125 and a follower gear 127. A partition wall is provided between a pair of roots rotors 102, 103 and a pair of screw rotors 105, 106 so that the byproducts from process chamber (not shown) are not transmitted directly to a pair of screw rotors 105, 106. In this conventional dry vacuum pump, shafts 114 a, 114 b respectively connected laterally to the middle of a pair of screw motor 105, 106 penetrates through the partition wall 108, and the penetrated portions of shafts are surrounded by a plurality of bearings 110 a, 110 b for smooth rotations of each shaft 114 a, 114 b. The opposing portions of each shaft 114 a, 114 b also are surrounded by a plurality of bearing 112 a, 112 b for the same. This patent is entirely incorporated into the present invention.

However, in a dry vacuum pump 100 disclosed in U.S. Pat. No. 5,549,463, as a partition wall 108 is formed between a pair of roots rotor 102, 103 and a pair of screw rotor 105, 106, the housing including these elements is divided into several portions, which causes to increase efforts and cost in manufacturing a dry vacuum pump 100.

Though, in a dry vacuum pump using screw rotor, a screw with changeable pitch is proposed without a partition wall to lower power consumption and increase the volume of the byproducts which is to be compressed and evacuated, larger rotor and pump housing than those in using a partition wall are necessary, which is evaluated to decrease efficiency.

In addition, in a dry vacuum pump as disclosed in aforementioned US patent, because bearings 110 a, 110 b supporting a pair of roots rotor 102, 103 and a pair of screw rotor 105, 106 are disposed on intake side which undergoes between vacuum and ambient pressure repeatedly on operation, grease for lubricating bearings 110 a, 110 b is leaked from bearings 110 a, 110 b due to the pressure difference, which causes troubles in pump. And also, in a dry vacuum pump as disclosed in U.S. Pat. No. 5,549,463, because bearings 110 a, 110 b are disposed on intake side, temperature thereon increases due to the pressure difference, friction and the like, which causes to shorten the lives of bearings 110 a, 110 b.

Meanwhile, a conventional dry vacuum pump comprises 4-5 roots rotors for lowering power consumption in operating, that is, for compressing more strongly the byproducts in gas state. The flow of the byproducts in roots rotor is illustrated in FIG. 8. Even though a partition wall between roots rotors is not illustrated in FIG. 8, but it must be understood that a partition wall is formed between them in practical. However, because the conventional dry vacuum pump using aforementioned 4-5 roots rotors indispensably comprises rotor housings, paired rotors, partition walls, and the like, it cause to increase numbers of elements and cost in assembling them. In addition, because the internal channels to inhale the byproducts in gas state and evacuate it are too long and complicated, it causes to increase factors of gas leakage and accumulate the byproducts thereon.

SUMMARY OF INVNETION

Accordingly, it is the first object of the present invention to provide an improved dry vacuum pump in which it is possible to reduce the power requirement and increase the volume of the byproducts generated in the process chamber to be compressed and evacuated without using a partition wall between a roots rotor end and a screw rotor end.

In addition, it is the second object of the present invention to provide an improved dry vacuum pump in which it is possible to lower the power consumption and increase the volume of the byproducts to be compressed and evacuated with using less roots rotors.

In addition, it is the third object of the present invention to provide an improved dry vacuum in which it is possible to protect dry pump's obstacle and lengthen bearing's life by changing the bearing's disposition.

To achieve the first object, a dry vacuum pump according to the first aspect of the present invention comprises:

(a) a cylindrical housing formed with the intake on one side for inhaling the object substance and with the outtake on opposing side for evacuating the object substance;

(b) a roots rotor embedded within the housing in communication with said intake;

(c) a screw rotor embedded within the housing and disposed closely to the roots rotor;

(d) a shaft fixed through the middle between said roots rotor and said screw rotor, and fixed rotatively to the housing in sealed state from the exterior; and,

(e) a driving motor installed outside the housing, to drive said roots rotor and said screw rotor for rotation in connection with said shaft,

wherein, a space is formed in the portion connected to said roots rotor among through the under sides of said roots rotor and said screw rotor, for staying the object substance.

To achieve the second object of the present invention, a dry vacuum pump according to the second aspect of the present invention comprises:

(a) a cylindrical housing formed with the intake on one side for inhaling the object substance and with the outtake on opposing side for evacuating the object substance;

(b) roots rotors embedded within the housing, and at least one embedded roots rotor in communication with said intake;

(c) a screw rotor embedded within the housing and disposed closely to at least one of the roots rotors;

(d) a shaft fixed through the middle between said roots rotors and said screw rotor, and fixed rotatively to the housing in sealed state from the exterior; and,

(e) a driving motor installed outside the housing to drive said roots rotors and said screw rotor for rotation in connection with said shaft,

wherein, a space is formed under side of one roots rotor of said roots rotors for staying the object substance, and the space is formed on the portion connected to said roots rotor among through the under sides of the other roots rotor of roots rotors and said screw rotor for staying the object substance, and the space formed under side of said roots rotor is communicated with upper side of said roots rotor through a predetermined fluid channel.

To achieve the third object, a dry vacuum pump according to the third aspect of the present invention comprises:

(a) a cylindrical housing formed with the intake on one side for inhaling the object substance and with the outtake on opposing side for evacuating the object substance;

(b) a roots rotor embedded within the housing in communication with said intake;

(c) a screw rotor embedded within the housing and disposed closely to the roots rotor;

(d) a shaft fixed through the middle between said roots rotor and said screw rotor, and fixed rotatively to the housing in sealed state from the exterior;

(e) a driving motor installed outside the housing to drive said roots rotor and said screw rotor for rotation in connection with said shaft;

(f) a rotation member to fix rotatively one end of the shaft which is connected to said roots rotor to one end of the housing; and,

(g) a bearing mechanism fitted on said shaft, and disposed on the outtake and the opposing end of the housing to smooth the rotation of said shaft.

Meanwhile, a term used in present description and claims “object substance” is to be understood that it includes gaseous substance and/or byproducts generated in process chambers of a semiconductor manufacturing device, a display manufacturing device and the like.

In addition, the term “the first” and “the second” in expression of “the first roots rotor”, “the second roots rotor”, “the first sump”, and “the second sump” is to be understood that it only means the sequences in which the object substance follows. The term “front end side”, unless described otherwise, is to be understood in a dry vacuum pump according to the present invention that it indicates the intake side for inhaling the object substance, instead of the outtake side for evacuating the compressed object substance. The term “rear end side” also is to be understood that it indicates the outtake side instead of the intake side.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to aid in understanding the invention and are incorporated into and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principles of the invention. In the drawings:

FIG. 1 is a schematic sectional view showing main parts of a dry vacuum pump according to the first aspect of present invention;

FIG. 2 is a partial view showing the internal parts in a dry vacuum pump in FIG. 1;

FIG. 3 is a view showing roots rotor's operation principle used in the present invention;

FIG. 4 is a view showing an alternative example in a dry vacuum pump according to the first aspect of the present invention wherein on both sides of roots rotor, installed on the intake, where a screw rotor is coaxially connected to a roots rotor;

FIG. 5 is a sectional view showing main parts of a dry vacuum pump according to the second aspect of present invention;

FIG. 6 is a sectional view showing a dry vacuum pump according to the third aspect of present invention;

FIG. 7 is a sectional view showing a conventional dry vacuum pump; and,

FIG. 8 is a view showing roots rotor's operation used in a conventional dry vacuum pump in which a few roots rotors are included in one dry vacuum pump.

DETAILED DESCRIPTION OF INVENTION

The following embodiments are given for the purpose of illustration only and are not intended to limit the scope of this invention.

Hereinafter, the preferred embodiments according to the present invention will be described with reference to the accompanying drawings. Here, when one element is connected to another element, one element may be not only directly connected to another element but also indirectly connected to another element via another element. Further, irrelative elements are omitted for clarity. Also, like reference numerals refer to like elements throughout.

FIG. 1 is a sectional view showing main parts of a dry vacuum pump according to the first aspect of present invention.

Referring to FIG. 1, a dry vacuum pump 1 according to the first aspect of the present invention comprises a roots motor 14 on front end side, a driving motor 26, preferably water-cooling driving motor on rear end side, and a screw motor 18 between the roots motor 14 and the driving motor 26. The screw motor 18 is coaxially connected to the roots rotor 14 with the aid of a shaft 24. In one alternative embodiment, the screw motor 18 can be coaxially connected to the roots rotor 14 without the aid of a shaft 24. In another alternative embodiment, the roots motor 14 and the screw motor 18 are manufactured integrally or assembled them by welding after manufacturing them individually. Other alternative connecting methods except the aforementioned will be considered by a person having ordinary skill in the same art.

The roots rotor 14 and the screw rotor 18 are installed within the cylindrical housing 10. In the housing 10, the intake 12 to inhale object substance into the vacuum pump 1 is formed on upper side of the roots rotor 14 in drawings. Because this intake 12 acts to inhale the object substance within process chamber (not shown) of semiconductor or display manufacturing devices into the vacuum pump 1, it is directly connected to the process chamber in sealed state. Entirely, a cylindrical housing 10 including said intake 12 is connected to the process chamber of semiconductor or display devices in sealed state and protected in sealed state so that exterior substance does not to enter into the housing. And also, the portion penetrated by the shaft 24 is protected in sealed state from outside. The object substance inhaled through the intake 12 is trapped between lobes 14 a, 14 b installed in roots rotor 14 by rotation of roots rotor 14, and moved toward opposing side of the intake 12 (Refer to FIGS. 1 and 2).

After the object substance inhaled into the vacuum pump 1 with the rotations of roots rotor 14 is stayed temporally on the space 16 (Hereinafter called “powder sump”) formed on through underside of roots rotor 14 and a portion of underside of screw rotor 18, it is directed toward screw rotor 18 with pressure applied by the roots rotor 14. The powder sump 16, as illustrated in drawing, occupies most space of underside of roots rotor 14 and a part space of underside of screw motor 18. The powder sumps 16 formed underside of roots rotor 14 and underside of screw rotor 16 are communicated and thus forms one space. This powder sump 16 can eliminate necessities of the partition wall which has been used for lowering power consumption and increasing the volume of the object substance, particularly

the object substance to be compressed and evacuated in gas state in the conventional dry vacuum pump. In addition, the powder sump 16 can keep the foreign solid substance to stay on the space, which causes to prevent the breakage of the screw rotor 18.

The object substance entered by force into screw rotor 18 via said powder sump 16 is compressed and evacuated through a outtake 20 formed on rear end side of a vacuum pump 1 by the rotation of the screw rotor 18 in one direction and the pressure transmitted from a prior step.

Said shaft 24 mounted through the cylindrical housing 10 is supported on the front side wall 28 and the rear side wall 30, respectively, of the vacuum pump with aid of bearings 22 a, 22 b, 22 c disposed as illustrated in FIG. 1. The shaft 24 illustrated right side in drawing is connected to the driving motor 26, particularly water-cooling motor 26, and rotated by motor's operation.

In FIG. 2, the housing 10 inside the dry vacuum pump 1 as illustrated in FIG. 1 is illustrated schematically. As well understood from this drawing, in the dry vacuum pump 1 according to the first aspect of the present invention, the powder sump 16 is formed commonly on underside of roots rotor 14 and a portion of underside of screw rotor 16 connected to roots rotor 14, and acts to let the object substance which is transmitted to the under side of roots rotor 14 by the driving of roots rotor 14, to stay thereon temporally and directed toward the screw rotor 18, therefore a partition wall required in the conventional dry vacuum pump is not necessary. Though the screw rotor 18 with same pitch is illustrated in drawing, for increasing compression's rate of gaseous substance and/or the byproducts, the screw rotor 18 with different pitch, that is, in which lengths of pitch get shorter and shorter from the intake 12 to the outtake 20, can be installed.

FIG. 3 is a view showing roots rotor's operation principle according to a preferred embodiment of the present invention.

Referring to FIG. 3, the object substance which is inhaled into the inside of the dry vacuum pump 1 according to the first aspect of the present invention is trapped between lobes 14 a, 14 b, 14 c by the rotation of the roots rotor 14 as illustrated in drawing and conveyed to a predetermined open space or a following process space. In the present invention, the object substance is conveyed to the powder sump 16 formed commonly on underside of roots rotor 14 and a portion of underside of screw rotor 16, and then directed to the screw rotor 18 through the powder sump 16 formed on the roots rotor 14. The operation principle of roots rotor itself is well known to the person having ordinary skill in the same art.

FIG. 4 is a view showing an alternative example in a dry vacuum pump according to the first aspect of the present invention.

In a dry vacuum pump according to the alternative embodiment, though it is as same as the aforementioned dry vacuum pump according to the preferred embodiment of the present invention in that the object substance and/or the byproducts in gas state is trapped on space between lobes of roots rotor 14 and conveyed to the powder sump 16′ formed on under side of the same, but it is different in that the screw rotor is installed on both sides of the roots rotor 14, and said powder sump 16′ is communicated with a portion of the screw rotor and thereby the object substance in gas state and/or the byproducts are directed to opposing directions. In this alternative embodiment, because the object substance in gas state and/or the byproducts are directed toward the screw rotor installed on both side of the roots rotor 14, that is, opposing directions, the outtakes (not illustrated) are formed on both sides. In addition, the rotation of the screw rotor installed on both sides of the roots rotor 14 is performed by one shaft 24, and the conveyance direction of the byproducts is dependent on the outtake's position. That is, the screw rotor illustrated right side in drawing is installed to direct the byproducts right and the screw rotor illustrated left side in drawing is installed to direct the byproducts left. Other alternative parts or elements caused from the above alternation in configuration can be considered easily by the person having ordinary skill in the same art.

As the dry vacuum pump 1 according to the first aspect of the present invention does not include a partition wall between the roots rotor 14 and the screw rotor 18, the increase in the number of elements caused from the partitioning of the housing is not occurred and also damage to the screw rotor is not made.

In FIG. 5, the main parts of a dry vacuum pump according to the second aspect of present invention is illustrated. The dry vacuum pump 21 according to the second aspect of the present invention is similar to the dry vacuum pump 1 according to the first aspect of the present invention in most parts, but it is different in that at least two powder sumps 15, 16 are formed and a fluid channel 8 is formed between the first roots rotor 13 and the second roots rotor 14 as illustrated in FIG. 5. Hereinafter, the configurations of the dry vacuum pump 21 according to the second aspect different from those of the dry vacuum pump 1 according to the first aspect will mainly described.

Referring to FIG. 5, the first and second roots rotor 13, 14 and the screw rotor 18 are embedded in a housing 10. In the housing 10, the intake 12 for inhaling the object substance into the dry vacuum pump 1 is formed on upper side of the roots rotor 13 in drawing. Because the object substance within process chamber (not illustrated) of the semiconductor or display manufacturing device is to be inhaled to inside the vacuum pump 1 through the intake 12, the intake 12 is directly connected to the process chamber (not illustrated) in sealed state. Entirely, the cylindrical housing 10 including said intake 12 is connected to the process chamber of the semiconductor or display manufacturing devices in sealed state and protected in sealed state so that the exterior substance does not enter into. And also, the portion penetrated by the shaft 24 is protected in sealed state from outside. The object substance inhaled through the intake 12 is trapped between lobes (alternative lobes 14 a, 14 b of the roots rotor 14 in FIG. 2) formed on roots rotor 13 by the rotation of the first roots rotor 13, and moved toward opposing side of the intake 12 (Refer to FIG. 5 together with FIG. 2).

After the object substance inhaled into the vacuum pump 1 with the aid of the first roots rotor 13 is arrived to the predetermined space 15 (Hereinafter called “first powder sump”) by the rotations of roots rotor 13 and stayed temporally thereon, it is conveyed to the upper side of the second roots rotor 14 via the fluid channel defined by the partition wall 4 opened under side and the partition wall 6 opened upper side. This fluid channel 8 can substitute a plurality of, for example 4-5, roots rotor and partition walls therebetween which have been used in the conventional dry vacuum pump.

As the object substance conveyed to the upper side of the second roots rotor 14 is trapped between the lobes 14 a, 14 b formed on the second roots rotor 14 by the rotation of the second roots rotor 14, and conveyed to the opposing side of the intake 12 (refer to FIG. 5 together with FIG. 2), the object substance is conveyed to the predetermined space 16 (Hereinafter called “second powder sump”) formed commonly on the under sides of the second roots rotor 14 and the screw rotor 18, and then directed toward the screw rotor 18 with pressure applied by the second roots rotor 14. The second powder sump 16 occupies most space of the underside of the second roots rotor 14 and a part space of the underside of screw motor 18. The powder sump 16 formed on the underside of roots rotor 14 and formed on the underside of the screw rotor 18 is communicated and thus forms one space.

The object substance entered by force into screw rotor 18 via the second powder sump 16 is compressed and evacuated through the outtake 20 formed on rear end side of the vacuum pump 1 by the rotation of the screw rotor 18 in one direction and the pressure transmitted from a prior step.

Said shaft 24 installed through the cylindrical housing 10 is supported on the front side wall 28 and the rear side wall 30, respectively, of the vacuum pump with the aid of bearing mechanism 22 a, 22 b, 22 c. The shaft 24 illustrated right side in drawing is connected to the driving motor 26, particularly water-cooling motor 26, and rotated by the motor's operation.

Whether it is the object substance which is inhaled into the inside of the dry vacuum pump 21 according to the second aspect of the present invention through the intake 12, or it is the object substance which is conveyed to the second roots rotor 14 with the aid of the first roots rotor 13 aforementioned, it is trapped between the lobes 14 a, 14 b, 14 c by the rotation of the first or second roots rotor 13 or 14, and conveyed to the predetermined space or the space of a following step.

In FIG. 6, the sectional parts of the dry vacuum pump according to the third aspect of present invention is illustrated. The dry vacuum pump according to the third aspect of the present invention is as almost same as the dry vacuum pump according to the first and second aspects of the present invention except the disposition of a bearing mechanism. Hereinafter, the configurations of the dry vacuum pump according to the third aspect different from those of the dry vacuum pump according to the first and second aspects will be mainly described.

Referring to FIG. 6, the dry vacuum pump 31 according to the third aspect of the present invention, in addition to individual or common elements which are included in the dry vacuum pump according to the first and second aspects of the present invention, further comprises a rotation member 27 for fixing rotatively one end of the shaft 24 connected the roots rotor 14 to one end of the housing 10, and the bearing mechanism 22 d, 22 e which are fitted on shaft 24 and disposed on the outtake side 20 and the opposing end side of the housing 10 to support the shaft 24 and smooth the rotation of the shaft 24.

The shaft 24 is installed through the housing 10 as described the above, and when the roots rotor 14 and the screw rotor 18 connected to the shaft 10, respectively, are rotated, the rotation member 27 to which one end of the shaft 24 is fixed to support the rotating shaft 24, and the bearing mechanism 22 d, 22 e to support the shaft 24 can be associated.

The rotation member 27 acts to fix rotatively one end of the shaft 24 connected the roots rotor 14 which is disposed on intake side of the housing to one end of the housing 10, and is connected by a pin or bolts.

A finish wall 29 is also formed on one end of the housing 10 to which the rotation member 27 is fixed. The finish wall 29 acts to prevent the separation of the pin member 27 and support the shaft 24 more safely in case that the shaft 24 is fixed rotatively to the housing by using a pin member.

The plural bearings 22 d, 22 e are disposed on the shaft 24 to support the shaft 24 and smooth the rotation of the shaft 24.

The first bearing mechanism 22 d surrounds the part of the shaft 24 disposed on an end of the screw rotor 18 running to the outtake 20 of the housing 10, and helps the rotation of the shaft 24. The second bearing mechanism 22 e is disposed on one end of the shaft 24 connected to the driving motor 26, and helps the rotation of the shaft 24.

The second bearing mechanism 22 e is disposed on one end of the shaft 24 elongated to one end of the housing 10 to prevent obstacles from the second bearing 22 e disposed on the shaft 24 running to the intake 12 of the housing 10.

Meanwhile, a partition wall (not illustrated) may be also formed between the screw rotor 18 and the roots rotor 14. In case that the partition wall (not illustrated) is used, the partition wall (not illustrated) is formed to support the residual part of the screw rotor 18 and roots rotor 14 excluding the shaft 24.

In addition, in the dry vacuum pump according to the third aspect of the present invention, for staying the object substance, the space 16 (“powder sump”) is formed on the portion connected to roots rotor 14. The powder sump 16 functions as same as the dry vacuum pump according to the first and second aspect of the present invention.

Under the configuration as described the above, as the bearing on the intake of the housing can be omitted in the dry vacuum pump according to the third aspect of the present invention, it makes the configuration simple and production easy, and thereby increase production efficiency and lengthen the bearing life.

Though the dry vacuum pump is described referring to the preferred embodiment according to each aspect of the present invention, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. 

1. A composite dry vacuum pump comprising: a cylindrical housing formed with the intake on one side for inhaling the object substance and with the outtake on opposing side for evacuating the object substance; a roots rotor embedded within the housing in communication with said intake; a screw rotor embedded within the housing and disposed closely to the roots rotor; a shaft fixed through the middle between said roots rotor and said screw rotor, and fixed rotatively to the housing in sealed state from the exterior; and a driving motor installed outside the housing, to drive said roots rotor and said screw rotor for rotation in connection with said shaft, wherein a space is formed on the portion connected to said roots rotor among through the under sides of said roots rotor and said screw rotor, to stay the object substance.
 2. The dry vacuum pump according to claim 1, wherein the space formed under side of said screw rotor is larger than the space formed under side of said roots rotor.
 3. The dry vacuum pump according to claim 1, wherein said driving motor is a water-cooling driving motor.
 4. The dry vacuum pump according to claim 1, wherein a pitch of said screw rotor is get shorter and shorter from the intake to the outtake.
 5. A composite dry vacuum pump comprising: a cylindrical housing formed with the intake on one side for inhaling the object substance and with the outtake on opposing side for excavating the object substance; a roots rotor embedded within the housing, and at least one roots rotor in communication with said intake; a screw rotor embedded within the housing and disposed closely to at least one of the roots rotor; a shaft fixed through the middle between said roots rotor and said screw rotor, and fixed rotatively to the housing in sealed state from the exterior; and, a driving motor installed outside the housing, to drive said roots rotor and said screw rotor for rotation in connection with said shaft, wherein, space is formed under side of one roots rotor of said roots rotor to stay the object substance, and the space is formed on the portion connected to said roots rotor among through the under sides of the other roots rotor of roots rotor and said screw rotor, to stay the object substance, and the space formed under side of said roots rotor is communicated with upper side of said roots rotor through the predetermined fluid channel.
 6. The dry vacuum pump according to claim 5, wherein the space formed under side of said screw rotor is larger than the space formed under side of said roots rotor.
 7. The dry vacuum pump according to claim 5, wherein said driving motor is a water-cooling driving motor.
 8. The dry vacuum pump according to claim 5, wherein a pitch of said screw rotor is get shorter and shorter from the intake to the outtake.
 9. A composite dry vacuum pump comprising: a cylindrical housing formed with the intake on one side for inhaling the object substance and with the outtake on opposing side for excavating the object substance; a roots rotor embedded within the housing in communication with said intake; a screw rotor embedded within the housing and disposed closely to the roots rotor; a shaft fixed through the middle between said roots rotor and said screw rotor, and fixed rotatively to the housing in sealed state from the exterior; a driving motor installed outside the housing, to drive said roots rotor and said screw rotor for rotation in connection with said shaft; a rotation member for fixing rotatively one end of the shaft which is connected to said roots rotor to one end of the housing; and bearing mechanism fitted on said shaft and disposed on the outtake and opposing end of the housing to smooth the rotation of said shaft.
 10. The dry vacuum pump according to claim 9, wherein the space formed on the portion connected to said roots rotor among through under sides of said roots rotor and said screw rotor. 